Oligocene Laccoliths and Sedimentary Rock Domes of the Henry Mountains
United States of America
The diorite porphyry laccolithic intrusion of Mount Hillers, southern Henry Mountains, Utah, is surrounded by upturned beds of strata and interleaved diorite sills dipping 75°–85°.
Geological Period
Oligocene
Main geological interest
History of geosciences
Tectonics
Location
Colorado Plateau, United States of America
37°53’16”N, 110°41’51”W
The diorite porphyry laccolithic intrusion of Mount Hillers, southern Henry Mountains, Utah, is surrounded by upturned beds of strata and interleaved diorite sills dipping 75°–85°.
The laccolithic structures identified in 1875 by G.K. Gilbert as important magmatic contributions to tectonic processes of mountain building.
Mountain building is a seminal research topic of tectonics with great significance for society, providing dramatic landscapes with profound effects on ecosystems, climate, and human activities. The dome-like structures in Triassic to Jurassic sedimentary rocks of the Henry Mountains offer the defining locality (Gilbert, 1877) and one of the best exposures of laccoliths worldwide. Gilbert’s scientific methodology, developed while he investigated these structures, combines field observations with mechanical principles to set the example for those who followed (Pollard and Johnson, 1973; Jackson and Pollard, 1990), but it also provides an educational template for students of structural geology (Pollard and Martel, 2020).
View of Mount Holmes, where G. K. Gilbert on August 22, 1875 developed the concept of a laccolithic intrusion that lifts and bends overlying strata.
- Geological description
During exploration of the Henry Mountains in 1875, G.K. Gilbert (1843-1918) postulated a process through which magma rose through earth’s lithosphere in a dike, spread laterally as a sill, and opened a thick chamber by lifting and bending the overlying strata. He gave these chambers of ‘molten rock’ the name “laccolite” (now, laccolith) and described associated dikes, uplifted sills, and domed sedimentary strata (Gilbert, 1877; Hunt, 1988). Geological maps of the southern Henry Mountains (Mount Holmes, Mount Ellsworth, and Mount Hillers) show the laccolithic intrusions are circular in plan and formed at similar depths, 3-4 km. The overlying sedimentary rock domes have similar radii, 5–7 km, but their amplitude increases from 1.2 to 1.8 to 2.5 km, respectively. Limb dips steepen from 20° to 50°–55° to 75°–85°, and thus record serial stages in the deformation of strata over the growing laccolithic intrusions (Jackson and Pollard, 1988). The Henry Mountains are the original and now classic examples of magmatic structures building mountains within a broader typology of mountain belts worldwide. Gilbert’s investigation exemplifies a scientific methodology that progresses from field observations to idealizations to physical principles to mechanical models to solutions tested using field data (Pollard and Martel, 2020).
- Scientific research and tradition
G.K. Gilbert proposed a mechanical model for laccoliths and tested it with field data (Gilbert, 1877). Others followed with computer modeling of faulting and fracturing over laccoliths (Jackson and Pollard, 1988, 1990). Doming over laccoliths is cited as a canonical problem for bending sedimentary strata (Pollard and Martel, 2020).
Gilbert’s conceptual model of an ideal laccolith with accompanying sills and dikes, conceived in the Henry Mountains in 1875, published in 1877. Diorite porphyry: black.
- Reference
Gilbert, G.K. (1877) Report on the geology of the Henry Mountains, Monograph. U.S. Government Printing Office. Available at: https://doi.org/10.3133/70039916.
Hunt, C.B. (1988) Geology of the Henry Mountains, Utah, as recorded in the notebooks of G. K. Gilbert, 1875–76. Geological Society of America. Available at: https://doi.org/10.1130/MEM167.
Jackson, M.D. and Pollard, D.D. (1988) ‘The laccolith-stock controversy: New results from the southern Henry Mountains, Utah’, GSA Bulletin, 100(1), pp. 117–139. Available at: https://doi.org/10.1130/0016-7606(1988)100<0117:TLSCNR>2.3.CO;2.
Jackson, M.D. and Pollard, D.D. (1990) ‘Flexure and faulting of sedimentary host rocks during growth of igneous domes, Henry Mountains, Utah’, Journal of Structural Geology, 12(2), pp. 185–206.
Pollard, D.D. and Johnson, A.M. (1973) ‘Mechanics of growth of some laccolithic intrusions in the Henry mountains, Utah, II: Bending and failure of overburden layers and sill formation’, Tectonophysics, 18(3), pp. 311–354. Available at: https://doi.org/10.1016/0040-1951(73)90051-6.
Pollard, D.D. and Martel, S.J. (2020) Structural Geology: A Quantitative Introduction. Cambridge University Press.
- Author(s)
Marie D. Jackson.
Department of Geology and Geophysics, University of Utah, USA.
David D. Pollard.
Department of Earth and Planetary Sciences, Stanford University, USA.
